A puncture was the most likely cause for Ayrton Senna's fatal accident at the 1994 San Marino Grand Prix, according to Williams chief designer at the time Adrian Newey.

The reason behind the accident has never been fully understood, with a wide range of theories from a broken steering column - which had been made longer at Senna's request by welding an extra piece onto it ahead of the race - to low tyre pressures due to a long period behind the safety car. But in a recent interview with the Guardian, Newey, who admitted that the aftermath of the accident affected him physically, said those theories do not necessarily stack up.

"For the whole team it was incredibly difficult," he told the newspaper. "I remember the day after the race was a bank holiday Monday and some of us came in to try and trawl though the data and work out what happened. They were dark weeks.

"The honest truth is that no one will ever know exactly what happened. There's no doubt the steering column failed and the big question was whether it failed in the accident or did it cause the accident? It had fatigue cracks and would have failed at some point. There is no question that its design was very poor. However, all the evidence suggests the car did not go off the track as a result of steering column failure.

"If you look at the camera shots, especially from Michael Schumacher's following car, the car didn't understeer off the track. It oversteered which is not consistent with a steering column failure. The rear of the car stepped out and all the data suggests that happened. Ayrton then corrected that by going to 50% throttle which would be consistent with trying to reduce the rear stepping out and then, half-a-second later, he went hard on the brakes.

"The question then is why did the rear step out? The car bottomed much harder on that second lap which again appears to be unusual because the tyre pressure should have come up by then - which leaves you expecting that the right rear tyre probably picked up a puncture from debris on the track. If I was pushed into picking out a single most likely cause that would be it."

Newey, who is now Red Bull's technical director, said the accident had made him question his career in motorsport.

"The little hair I had all fell out in the aftermath," he added. "So it changed me physically. It was dreadful. Both Patrick Head [Williams's technical director] and myself separately asked ourselves whether we wanted to continue in racing. Did we want to be involved in a sport where people can die in something we've created?"